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116 Cards in this Set
- Front
- Back
Corneal curvature: steep corneal curvature require?
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smaller lenses
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Corneal curvature: flat cornea requires?
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a larger lense
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Aligned on K, ie cornea is 45.00
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base curve is the same as the flat k of the cornea, base curve is 45.00
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Steep fit, ie cornea is 45.00
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base curve is steeper than flat k, base curve is 45.50
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Flatter fit, ie cornea is 45.00
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base curve is flatter than flat k, base curve is 44.50
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In determining final power there is 3 things you have to take into account
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1. patients rx
2. effectivity if the power is 4.00 or over in any meridian 3. Tear lens |
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Tear lens
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the tear filled gap created by the difference between the base curve and the corneal curve.
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SAM
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Steep add minus, there is a plus tear lens, so you need to add minus to cover it.
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FAP
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Flat add plus, there is a minus tear lens so you need to add plus to cover it.
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Center thickness
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thickness of the lens, minus lenses will be thinner in the middle than plus lenses. the higher the minus the thinner the center.
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Edge thickness
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0.08-0.12, a good thin edge adds comfort, the thicker the lens the less blinking which then leads to 3 and 9 o'clock staining.
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Axial edge lift
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the vertical distance from the lens edge to the extension of the base curve.
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Radial edge lift
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the extension of the lens edge perpendicular to the extension of the base curve, always about 80% of axial edge lift.
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edge clearance
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how much lift the lens has against the peripheral cornea. What you see with fluorescein pattern.
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If edge lift is too great you can
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steepen the peripheral curve radius, or narrow the peripheral curve width.
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The three basic RGP lens designs
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1. interpalpebral
2. Basic tri curve/tetra curve 3. korb or lid attachment design |
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Interpalpebral design
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original design, smaller lenses 8.0-8.6mm
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Basic tri-curve/tetra curve design
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everyday rgp design. usually fits under superior lid. not a lid attached design but can happen. dia 9.2-9.4mm, optic zones usually 7.7-8.2mm
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Korb or lid attachment design
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anterior front surface of periphery of lens attaches to the lid and moves with it.
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Selecting a diameter will depend on:
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pupil size, corneal curvature (flat corneas get larger lenses), palpebral aperature size
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lens adherence =
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lens adherence = optic zone diameter * radius of base curve. Increasing SAG increases adherence.
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If diameter increases and the BS remains constant?
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SAG increases, and lens adherence increases as well.
Lens adherence(SAG) = optic zone diameter * radius of base curve. |
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If sagitta increases and the diameter remains constant?
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the BC must steepen.
Lens adherence(SAG) = optic zone diameter * radius of base curve. |
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Eccentricity value
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how the cornea or CL flattens as you go out towards the periphery. Avg is 0.55, so 0.6 means the cornea flattens faster then normal, and 0.40 means the cornea flattens slower thena normal.
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base curve/optic zone interrelationship, in order to maintain a constant SAG.
if base curve is flattened by 0.25D |
then optic zone must be increased by 0.5.
Lens adherence(SAG) = optic zone diameter * radius of base curve. |
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base curve/optic zone interrelationship, in order to maintain a constant SAG.
if base curve if steepened by 0.25D |
then optic zone must be decreased by 0.5mm
Lens adherence(SAG) = optic zone diameter * radius of base curve. |
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Tri curve lens radius if base curve is 7.50mm
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secondary curve radius = 1.0 to 1.5mm flatter = 8.50mm
peripheral curve radius = 1.5 to 2.00mm flatter than SCR = 10.0mm or 10.5mm |
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Peripheral curve widths, overall diameter =?
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overall diameter = optic zone + 2(secondary curve width) + 2(peripheral curve width)
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Tetra curve lens peripheral curve system.
secondary curve radius SCR = ? Tertiary curve radius TCR =? Peripheral curve radius PCR =? |
SCR = BC + 0.8mm
TCR = SCR + 1.0mm PCR = TCR + 1.4mm |
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Tetra-curve widths
SCW = ? TCW = ? PCW = ? |
SCW = 0.3mm
TCW = 0.2mm PCW = 0.2mm |
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Center Thickness for a minus lens?
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center thickness = constant + ([BVP]/100)
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Center thickness for a plus lens?
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center thickness = constant + (2[BVP]/100)
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pros and cons of thin lenses
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pros:
more comfortable more oxygen transmissible good centration cons: more fragile more flexure more warpage fragile edges |
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most important consideration when choosing a lens
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patient's pupil size, because if pupil is bigger than optic zone flare might become and issue.
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RGP Panofocal
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front aspheric, doesn't correct residual astigmatism but it reduces aberrations to clear the image. cost more but work about 80% of the time. use panofocal when residual astigmatism is about 1.25d or less.
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Movement is important because
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1. tear exchange
2. flushing debris away 3. wetting the cornea 4. minimizing pressure on ocular tissues. |
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if lens has excessive movement what can you do?
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1. make sure patient is adapted to the lens, movement could be due to excessive tearing.
2. make the BC steeper 3. change the diameter, go smaller to give it less lid grab. 4. Taper the edges to create less lid grab. 5. change the lens thickness, going thinner will slow lens movement. |
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If lens movement is inadequate
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1. Flatten the bc, makes the lens looser
2. go to a larger diameter to increase lid grab and oz. 3. change edge design, last resort. |
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If lens is too low
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try a larger diameter lens with a larger optic zone and flatter base curve to get more superior lid grab
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if lens is too high
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try a smaller diameter lens with a steeper base curve or taper the edge to make the lid grab less.
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binding?
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lens is sucked onto the eye, patient maybe comfortable, but bad situation physiologically as it can harm the cornea.
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Fluorescein pattern
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assessing the relationship between the back of the lens and the front of the cornea including the tear lens
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static fluorescein pattern
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done without lids in play
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dynamic fluorescein pattern
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done with lids in play
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3 areas to observe while doing a fluorescein pattern
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1. central
2. para-central 3. peripheral or edge |
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WTR fluorescein pattern
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Dark H pattern
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ATR fluorescein pattern
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I pattern
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common sources of error on fluorescein patterns
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call mat up
Corneal topography Astigmatism Lids and lid action Lens construction Movement Amount of fluorescein used Tearing UV source Position |
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If you increase Optic zone diameter you must be sure to...
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flatten the base curve to maintain the same sagittal fitting relationship.
lens adherence(sag) = optic zone diameter * radius of base curve |
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If you decrease the optic zone diameter you must
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steepen the base curve.
lens adherence(sag) = optic zone diameter * radius of base curve. |
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Widening or flattening the peripheral curve
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more peripheral clearance will occur.
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Decreasing the optic zone diameter
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loosens the fitting relationship
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Axial edge lift
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vertical distance from the lens edge to the extension of the base curve of the lens, nothing to do with the cornea.
It is the measure of the linear distance between the edge of the CL and the projected trajectory of the BC of the lens. Usually you want this to be 0.10mm. Too high of an AEL will give you excessive clearance and you can get 3 and 9 o clock staining. |
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Radial edge lift
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linear distance between the edge of the cl perpendicular to the trajectory of the base curve. usually 0.08mm. Doesn't take cornea into concideration.
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Edge clearance
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the actual amount of lift of the lens off the cornea. The distance from the lens edge to the cornea measured perpendicular to the cornea. Should be less than the axial edge lift. This takes into account the cornea. This is what you're looking at when you do a fluorescein pattern.
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increasing the optic zone diameter
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the peripheral curve width decreases and the axial edge lift and edge clearance will also decrease, will result in a tighter fit.
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greater axial edge lift on an RGP may cause more
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3 and 9 o'clock staining due to the lid not moving over the lens property.
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Aspheric lenses
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ellipsoid, they parallel or simulate the flattening of the cornea better than regular spherical lenses. Broader contact zone. No worry about blending, and reduces flare.
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Most important aspect of lens design when determining comfort
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edge design, edge should be tapered and rolled. Want to ensure that the apex of the edge to be in the rear 1/3 of the lens and rolled. ideally 0.10mm
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Construction of the Korb lens
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1. thin to minimize mass and increase oxygen transmission.
2. larger lenses, so more of the lens can get under the lid 3. flatter fitting, 0.25-0.50 flatter than k, helps facilitate lift, ensures higher positioning. |
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Optimal diameter of a korb lens
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9.5 mm diameter with a 8.4 mm optic zone
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to raise a low riding lens
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use a korb lens or a minus lenticular
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minus carrier lenticular design
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makes the edge thicker making it easier to hook onto the upper lid.
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Lenticular flange
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front carrier radius or curvature is cut or lathed on to the edge of the lens
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Optic zone or front bowl of a lecticular
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central optical portion of the lens.
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The minus carrier radius or flange radius is made...
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1.0-3.0mm flatter than the base curve radius. The flatter the flange radius, the thicker the edge, the higher the lens rides and less stagnant it becomes.
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if the carrier radius is steeper in curvature than the central front radius of the lens
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then the lenticular is a plus edge lenticular.
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for more lift of the lens you would use a
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smaller bowl diameter, this allows the flange to be larger, allowing more plastic to get under the lid. The flange should ideally be 0.5-0.6mm.
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Junctional thickness
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determined where the front bowl meets the carrier. For plus lenses it is often the thinnest part of the lens, can break if too thin. Should not be thinner than 0.13-0.15mm or breakage can occur.
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Tapering for high minuses
medium tapers for power? heavier tapering for power? CN bevel for? plus lens lenticulars for? |
Tapering should be considered for -4.00 - -6.00 D
Medium tapers for -4.00 to -5.00D Heavy tapers for -5.00D to -6.00 D CN bevel for -6.00D to -8.00D plus lens lenticulars for > -8.00D |
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Albinism custom lenses help
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CL decreases searching nystagmus, a light tint can also help photophobia.
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X-chrom lens changes...
transmits? always worn on? |
color perception for anomalous trichromats.
590-700 non-dominant eye. enhances colors and depth perception, it oes not cure color vision defects. Only 20% of patients who try the lens actually order it even if it works. Lens it not good in dim illumination. |
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Custom lenses for arcus senilis
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when patients think that it looks like cancer. cosmetic by nature.
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Medical uses of CL's
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1. band keratopathy
2. corneal abrasion 3. bullous keratopathy 4. herpetic lesion |
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Sympathetic nervous system, high amount of stress
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patient wasn't sleeping, so excessive stimulation was causing both his eye to be dilated at all times. Used an opaque lens to mask the dialation
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Anisocoria
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fit both eyes with stock lenses, easier than fitting one eye with a custom lens that would match the other natural eye.
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Coloboma
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use opaque cl to cover coloboma.
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3 and 9 o'clock staining
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very common, almost a norm in RGP wearers.
Due to 1. thick edge 2. inferior positioning 3. irregular tear film 4. excessive edge lift, 1.0 is optimal, higher may lead to more incidence of staining |
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Vascularized Limbal Keratitis Stage I
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step beyond 3-9 o clock staining. heaping of corneal of limbal epithelium peripherally. This is due to insufficient edge lift.
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Lens adhesion
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seen in new extended wear RGP's. Happens during sleep, lens sits inferior temporal and you get an indention ring. Sulcus, compression ring, indention in the epithelium from RGP.
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Corneal dimpling
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depressions in the cornea due to CO2 bubbles under a tight zone. Looks like staining but not actually staining, epithelium is not broken, just more pooling in the depression.
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Epithelial microcysts
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wearing lenses too long, hypoxia. patient is in excruciating pain. during the day the cornea develops vesicles in the deep epithelium. over night these vesicles come to the surface burst and release nerve ending causing pain. Use bandage CL focus night and day silicone hydrogel.
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CCC central corneal clouding
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epithelial edema, typically in the area of the optic zone. gray haze. not adequate oxygenation.
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Edematous corneal formation ECF
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deeper more severe edema. more intense and longer lasting. resembles arborized tree branches, sort of resembles herpetic lesions. can last for weeks or even months. due to long term oxygen deprivation, will not stain because no breaks in epithelium. gradually wean the wearing schedule over a few weeks.
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corneal warpage syndrome
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avoid refitting with soft contact lenes. do not tell the patient to stop wearing PMMA lenses for 3 days before the exam.
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Rengsorff curves
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describes the change in corneal curvature and refraction after discontinuation of PMMA lens wear. Cornea gets much flatter in the days following discontinuation and then gradually goes back to normal.
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corneal topography mathematical method
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favored by visual scientists, an ellipse is closest model for optical portion of the cornea.
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Photokeratoscopy, corneal topography
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target or placido rings projected onto cornea and photographed with telecentric optical system. mail the picture to a team of people to calculate and analyze the k, take ~6 weeks to get results back
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Videokeratoscopy, corneal topography
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photographic camera replaced with video camera, analysis done in seconds and much more accurate.
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Topographical maps, corneal topography
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derived from radius measured at several thousand corneal positions.
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cataract surgery can leave the cornea
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with reduced sensitivity and can make patient unaware of corneal ulcer
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Aphakia patients have a huge problems with contact lenses in that they?
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tend to lose them, insurance will only cover the first pair after the surgery. Expect an aphakic to go through at least 4 lenses the first year alone.
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When to fit aphakes with CL's
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when you start obtaining reliable and repeatable refractions and k readings, indicating that the cornea has settled.
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welsh lens
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thin lens, fit steep and smaller to help and hold it to the eye. But it makes it hard to remove from a patient that has flaccid lids. Max thickness 0.35mm with a knife edge.
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Lenticular carriers in aphakia.
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better option.
dealing with a minus edge lenticular carrier lens. |
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Using glasses over contact lenses...
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1. over plus aphakic by 2-3D in Cl.
2. over plussed eye has increased mag, now compensate by putting 2-3d minus at the spectacle plane, the net affect here is minification, making the net magnification closer to that of the non-aphakic eye. 3. Opposite done to the normal eye, over minus then give plus at the spectacle plane to give a net magnification effect. |
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Little aphakes, congenital aphakes
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age 0 to 1 years old, too early to consider an IOL. But must do something before 6 weeks of development to allow proper development of visual system.
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Rx range of little aphakes
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range from 28-32d. tend to drop about 10d after the first year, due to the rapid growth of the eyeball. this is why an iol early is not a good option.
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k readings of little aphakes
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k readings will flatten over time, 40-60% over the first year.
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Monocular cataracts should be removed within?
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the first few days, this improves the chances that the child will not have amblyopia.
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Binocular cataracts should be removed within?
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the first few months.
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B&L lens used for congenital aphakes that B&L makes nearly no money on.
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Silsoft, primarily for children, DK 340. Always overplus a child because their world is within arm's length. This lens attracts lipids, must use miraflow cleaner.
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Why must lenses move? even if they have a dk of 340?
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1. pumps oxygen
2. pumps waste 3. wets cornea 4. without waste removal or corneal wetting the epithelium will break down and the eye can get infected. |
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IATS: Infant aphakic treatment study
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examines aphakic infants treated with IOL's versus no IOL's. Critical time is 6 weeks for babies with visual limitations, if intervention is needed do it before that time.
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Axial topography format
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data from points along any orneal meridian are always referenced to a normal axis running perpendicularly to the central cornea. may be of greater value in contact lens fitting, as there is more averaging of data over a larger area.
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Tangential topography format
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data from points along any corneal meridian are always referenced to their own individual radii of curvature. do not average data and are necesssary for refrative surgery where local variation of corneal curvature is critical to vision and laser reshaping.
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forme fruste
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mild minimal keratoconus that can go unnoticed for a while as VA can be 20/20 because steepening is only in the periphery.
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Sim K
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determined from power points on mires 7-9. 3 mm central zone.
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SAI or surface asymmetry index
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determined from the summed differences in corneal power between corresponding points 180 degrees apart on the mires over 128 equally spaced meridians on 4 central mires. Not highly correlated with best spectable VA, but can be used to monitor changes over time.
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SRI, or surface regularity index
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evaluates localized irregularity between adjacent rings over 256 hemi-meridians. SRI approaches zero for a perfectly smooth surface. High correlated with best spectable corrected visual acuity.
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Minimum center thickness
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0.10
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Minimum edge thickness
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0.10
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Junctional thickness
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0.13
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Range of DK/t values
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0-21%, can't go higher than 21% because that's the content in air.
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In daily wear lenses dk/t value should be
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6-8%
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In extended wear lenses DK/t value should be
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17-18%.
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